The social and political predicaments underpinning Europe’s Islamist challenge ominously resemble those that caused the decline of ancient Rome.

“You cry like a woman because you couldn’t defend like a man,” said Muhammad XII’s mother as the weeping emir left the Alhambra Palace for the ceremony in which he surrendered to Spain Islam’s last West-European realm.That was in 1492. Now the pendulum has swung. As Muslims this week again sent Christian Europe running for cover, the one shedding tears was European Union foreign policy chief Federica Mogherini. [..]

The attackers’ affiliation, motivation and aims became clear shortly after they murdered dozens and wreaked havoc in the EU’s capital: They were activated by Islamic State, which in turn explained through its news agency that it was out to attack “the Cross-bearing nations” – meaning Christendom – and that it has in store for them “black days” that will be much worse than what Europe has so far endured.

Historically, however, Europe and Islam have been at loggerheads intermittently since the eighth century, when Muslim armies conquered Spain and then invaded France through the Pyrenees before landing in Italy and reaching Rome.Consequent Muslim rule, from Barcelona to Sicily, may be trivia to current-day Christian Europeans, but to some Muslims it is a recollection both vivid and instructing. Similarly, the Ottoman conquests at Europe’s other end remain traumatic memories in Greece, Bulgaria, Serbia and Hungary. Now this legacy of disharmony is returning to the fore.

Yet unlike previous Muslim penetrations into Europe, which followed military conquests, the current presence follows a mostly peaceful immigration whose causes and results bring to mind not medieval Europe’s struggles with its Muslims but ancient Rome’s decline and fall.

BEFORE ROME was “delivered to the licentious fury of the tribes of Germany and Scythia,” as Edward Gibbon put it in The Decline and Fall of the Roman Empire, the number of Romans joining its army was steadily declining; the middle classes were overtaxed to finance grandiose public works; the cities that once were the empire’s social backbone were crumbling under the weight of foreign migrations; a hedonistic elite increasingly shunned politics; and a new religion’s believers threatened the social, political and cultural order.Much of this has been happening in Europe, gradually but steadily, in recent decades.

As long as this sort of historic perspective is not considered in the very Brussels that has just been targeted, Europe will remain on the defensive, its enemies’ gains will accumulate, and their Christian victims will grow increasingly restless, insecure and, ultimately, also violent.